Urinary dysfunction commonly caused by spinal cord injuries involves involuntary urinary retention, a condition which associated with urinary infections, renal damages or damages to the urinary tract. A common treatment of urinary retention is continuous or intermittent catheterization. Besides the inconvenience for the patient, catheters always represent a risk of acquiring infections. Alternatively suggested therapies include electric stimulation of the urinary bladder for providing muscle contraction and bladder emptying (see e.g. U.S. Pat. No. 6,393,323). Electric stimulation of the bladder needs consideration to that the urinary sphincter is stimulated to contraction by electricity and pulsed stimulation will become necessary which, however, may lead to uncontrolled squirts of urine through the urethra. It is obvious that there is a need for devices assisting with urinary bladder voiding which are efficient, reliable and that provide a high level of patient compliance.